The present invention relates to a method for preparing an aldehyde or alcohol by the reduction of a carboxylic acid or, more particularly, to a method for preparing an aldehyde or alcohol by the reduction of a corresponding carboxylic acid using a metal borohydride as the reducing agent.
When an aldehyde or an alcohol compound is desired to be prepared by the reduction of a corresponding carboxylic acid, it is a usual practice that the carboxylic acid is reduced as a solution in diethyl ether or tetrahydrofuran using lithium aluminum hydride LiAlH.sub.4 as the reducing agent. The applicability of this reducing method, however, is limited even by setting aside the problem of expensiveness of this reagent to probibit industrial application of the method. For example, reduction of an unsaturated carboxylic acid by this method is not always suitable for the preparation of the corresponding unsaturated aldehyde or alcohol since the reduction takes place not selectively at the carboxyl group alone but also the unsaturated linkage in the unsaturated carboxylic acid is also susceptible to reduction. When cinnamic acid is reduced in an ether solution by using lithium aluminum hydride as the reducing agent, for example, the reaction product is not the desired cinnamyl alcohol but dihydro-cinnamyl alcohol as is taught by R. F. Nystron, et al. in Journal of the American Chemical Society, volume 69, page 2548 (1947).
The unsaturated linkage in the unsaturated carboxylic acid may be protected from reduction by using a reducing agent system of lithium aluminum hydride combined with anhydrous aluminum chloride. For example, sorbic acid CH.sub.3 CH.dbd.CH--CH.dbd.CH--COOH can be reduced by this method into hexa-2,4-dienol CH.sub.3 CH.dbd.CH--CH.dbd.CH--CH.sub.2 OH. This method, however, is not practical due to the extremely low yield of the reaction product of, for example, only 20% in the above mentioned reduction of sorbic acid. Moreover, the reaction product in the reduction of a carboxylic acid using lithium aluminum hydride is always the corresponding alcohol while the corresponding aldehyde compound, which should have been formed at the intermediate stage, can hardly be obtained as the final product.
Sodium borohydride is less expensive as a reducing agent than lithium aluminum hydride and is used in various synthetic reactions. The reducing power of this reducing agent is, however, not strong enough to reduce a carboxylic acid into a corresponding aldehyde or alcohol. The reducing power of sodium borohydride can be strengthened by performing the reducing reaction in diglyme as the solvent by combining sodium boorhydride with anhydrous aluminum chloride as is taught by H. C. Brown et al. in Journal of the American Chemical Society, volume 75, page 6263 (1953) or by combining sodium borohydride with boron trifluoride BF.sub.3 as is taught by G. R. Pettit, et al. in Journal of Organic Chemistry, volume 27, page 2127 (1962).
These improved methods for increasing the reducing power of sodium borohydride are still not quite satisfactory due to the use of a Lewis acid such as anhydrous aluminum chloride and boron trifluoride. Namely, the procedure of the reaction is usually troublesome and the reaction is sometimes accompanied by undesirable side reactions when the product is not sufficiently stable resulting in decrease in the yield of the desired product. The selectivity of the reducing reaction is also not without problems. In addition, the diglyme used as the solvent can be recovered with some difficulties and the waste material containing aluminum chloride may be responsible for very serious environmental pollution when the waste water is discharged to public waterway without appropriate treatment of sewage disposal.
Lithium aluminum hydride and sodium borohydride both belong to a class of typical complex compounds capable of reducing various kinds of organic compounds by releasing hydrogen in the form of hydride ions. The problems in these hydrides as an industrial reducing agent are the expensiveness of lithium aluminum hydride having a relatively high reducing power and the relatively low reducing power of the less expensive sodium borohydride necessitating combined use of an auxiliary reagent to cause complicacy of the process and low yield of the product. At any rate, each of these hydrides is not suitable as a reducing agent when a carboxylic acid should be reduced to the corresponding aldehyde and not to the alcohol.